Vehicle Emissions fall into five main categories:
Carbon Dioxide (CO2), which is an inevitable product of burning a fuel which contains carbon (as all petroleum products do). CO2 does not pollute the air we breathe, but it is a main contributor to Global Warming and therefore has to be reduced. This means either using fuels containing less (or no) carbon (see the section on Alternative Fuels), or making vehicles and their engines more efficient – or both.
Carbon Monoxide (CO), which is produced when a carbon-based fuel is burnt incompletely. In high concentrations it is poisonous and has to be controlled. It can be reduced by more efficient combustion in the engine (so that CO2 is produced instead of CO) and further reduced by oxidising after combustion, in a Catalytic Converter. [2xCO + O2 = 2xCO2]
Hydrocarbons (HC), also known as “Volatile Organic Compounds (VOC) are really unburned fuel. They can be a problem to people with breathing difficulties and are a contributor to “Photochemical Smog” in certain climatic conditions. They can be reduced by more efficient combustion in the engine and further reduced by oxidising after combustion, in a Catalytic Converter. . [4HxCy + (x+4y)O2 = 2xH2O + 4yCO2]
Oxides of Nitrogen (NOx) are produced when air (which is mainly a mixture of Nitrogen and Oxygen) is heated as it is in an engine. NOx is a contributor to both Photochemical Smog and Acid Rain and can be an irritant to the lungs. Unlike CO and HC is cannot be removed by oxidation. The opposite process – the removal of Oxygen, known as “Reduction” is necessary to convert it back to Nitrogen and Oxygen.
Particulate Matter (PM) is very small particles, mostly of unburnt Carbon.
The first reductions in Vehicle exhaust emissions were achieved by improving engine design and tuning, but to reach the massively reduced levels of today, new add-on technologies had to be developed to attack specific pollutants and remove or chemically convert them. Often these had to be different devices, or combinations of devices, for the different kinds and sizes of engine.
Achieved Emission Reductions
As a result of the massive improvements in new vehicle emissions, driven not only by the increasingly tough emissions regulations, but by customer demand and by the genuine desire of Auto manufacturers to be part of the solution, not the problem, the pollution emitted by road vehicles has dramatically reduced.
Here are a few examples:
This graph using data from the European Auto-Oil II programme shows the predicted reduction of all pollutants from a base of 1995.
In the USA, road-side emission checks on vehicles as they pass, show that emissions have reduced drastically and those from newer vehicles are becoming almost negligible.